Fuel cells are new energy storing systems in which chemical energy is converted into electrical energy through electrochemical reactions of fuel and oxygen. Since they are not based on the Carnot cycle, they are potential cleaner energy sources that have higher theoretical power generation efficiency than sources of energy using fossil fuels. Additionally, fuel cells do not discharge a significant amount of environmental contaminants. Such fuel cells can be used as power sources for small electric/electronic devices, particularly portable devices, as well as for industrial, domestic, and transportation applications.
Fuel cells can be classified into molten carbonate fuel cells which operate at higher temperatures of approximately 500° C. to 700° C., phosphoric acid electrolyte cells which operate at approximately 200° C., and alkaline electrolyte fuel cells and polymer electrolyte membrane (PEM) fuel cells which operate at room temperature or at approximately 100° C. The working temperatures and constituent materials of fuel cells are determined by the type of electrolyte used in a cell.
Depending upon how the fuel is supplied to the anode, fuel cells can be classified into an external reformer type where fuel is supplied to the anode after being converted into a hydrogen enrichment gas by a fuel reformer, and a direct fuel supply type or internal reformer type where fuel in gaseous or liquid state is directly supplied to the anode.
An example of direct liquid fuel supply type fuel cells is a direct methanol fuel cell (DMFC). DMFCs generally use an aqueous methanol solution as a fuel, and a polymer electrolyte membrane with hydrogen ionic conductivity as an electrolyte. Since DMFCs do not require an external reformer and use fuel that is convenient to handle, they have the highest potential for use as portable energy sources.
Electrochemical reactions occurring in a DMFC are as follows: fuel is oxidized at the anode, and oxygen is reduced into water through a reaction with hydrogen ions at the cathode.
Anode Reaction: CH3OH+H2O→6 H++6 e−+CO2 
Cathode Reaction: 1.5 O22+6 H++6e−→3 H2O
Overall Reaction: CH3OH+1.5 O2→2H2 O+CO2 
As shown above, one methanol molecule reacts with one water molecule at the anode, to produce one carbon dioxide molecule, six hydrogen ions, and six electrons. The produced hydrogen ions migrate to the cathode through a polymer electrolyte membrane where they react with oxygen and electrons, which are supplied via an external circuit, to produce water. Summarizing the overall reaction in the DMFC, water and carbon dioxide are produced through the reaction of methanol with oxygen. As a result, a substantial part of the energy equivalent to the heat of combustion of methanol is converted into electrical energy.
The polymer electrolyte membrane with hydrogen ionic conductivity acts as a path for the hydrogen ions generated through the oxidation reaction at the anode to migrate to the cathode, and as a separator between the anode and the cathode. The polymer electrolyte membrane requires sufficiently high ionic conductivity to facilitate a rapid migration of a large number of hydrogen ions, electrochemical stability, and mechanical strength suitable for a separator, thermal stability at working temperature, ease of processing into a thin film so that its resistance to ionic conduction can be lowered, and a non-swelling property when permeated by liquid.
Fluorinated polymer membranes such as Nafion (Dupont, Wilmington, Del.), Assiflex (Asahi Chemicals, Japan), and Flemion (Asahi Glass, Japan) are currently available polymer membranes for a fuel cell. These fluorinated polymer membranes operate relatively well at low temperatures, but lose water contained therein at higher temperatures of at least 130° C., thereby causing destruction of an ion channel structure which affects ionic conductivity. In the case of DMFC, methanol leakage through the membrane occurs and its practicality is low. Also, due to its high costs, the fluorinated polymer membrane is difficult to commercialize.
There has been intensive research aimed at developing a less expensive polymer membrane than Nafion, such as a trifluorostyrene copolymer disclosed in U.S. Pat. No. 5,422,411 in order to overcome these disadvantages. However, the less expensive polymer membrane has poor mechanical properties and film forming ability.
Additionally, U.S. Pat. No. 6,245,881 also discloses various sulfonated polyimides prepared by copolymerizing diamine monomers containing a sulfonic group. These sulfonated polyimides have thermal stability and oxidation/reduction stability much higher than polymeric materials of conventional ion exchange membranes. However, the diamine monomers containing a sulfonic group are restricted and are not well dissolved in general solvents except for m-cresol. In addition, a degree of polymerization is low due to its relatively low reactivity, and thus the formation of a film does not smoothly occur.